If you have identified a bug in GiNaC you are welcome to send a detailed bugreport to the ginac-list mailing list.
Please think about your bug! This means that you should consider including a short description of your operating system and compiler with corresponding version numbers in case this might be relevant. A description of your configuration options may also be helpful. If it is a systematic bug in the library, a short test program together with the output you get and the output you expect will help us to reproduce it quickly. Also, do not forget to report the GiNaC version number and do not even think of sending core files. Obviously, the usual rules how to report bugs effectively all apply.